Implantable medical devices take many forms to provide therapy to a patient. For example, implantable medical devices provide pacing therapy to assist in maintaining proper heart rhythm. Pacing therapy may be applied to treat a number of conditions, such as atrial fibrillation, atrial tachyarrhythmia, atrial arrhythmias, ventricular fibrillation, ventricular tachyarrhythmia, and bradyarrhythmias caused by heart block or sinus node dysfunction. Implantable medical devices may also take the form of an implantable cardioverter defibrillator to provide therapy for sudden cardiac arrest, ventricular tachycardia, and ventricular fibrillation.
In order to provide therapy, an implantable medical device (“IMD”) communicates with a plurality of implantable slave devices, such as sensors, to monitor conditions including heart electrical activity and blood oxygen content, for example. Both the sensors and the implantable medical device are implanted within the body, and in order to reduce interference to the body, the implantable medical device communicates with the sensors over a small bus having a minimum number of electrically conductive wires. For example, communication information, along with power and ground, is provided over two conductive wires. At the same time, the implantable medical device operates by way of an internal power source, usually in the form of a battery, which has a limited amount of available power. Moreover, because replacement of the implantable medical device requires surgery to the patient, conservation of power is an important consideration.
Implantable medical devices are often required to deliver therapy in the form of electrical stimulation to the patient. Accordingly, reliable communication between the implantable medical device and the implantable sensors external to the IMD is important. Furthermore, since physical characteristics such as reduced size and space of the devices are desired in order to decrease patient stress, simplify input procedures, and reduce surgical complications, such characteristics are therefore important considerations that need to be addressed. Since the sensors are often required to be placed within the heart, the amount of available space for additional circuitry, power and memory is limited.